Optical fibre manufacture

ABSTRACT

In an inside tube vapor phase deposition process for the production of doped silica glass by an oxidation reaction for optical fibre manufacture, in particular fluorine doped silica, the oxidation reaction is prevented from occurring until the reactant material, for example silicon tetrachloride and a fluorinating reagent, has been heated to a temperature above that required for the oxidation reaction. This preheating of the reactant material results in the oxidation reaction producing compositions of glasses which are not allowed from thermodynamic considerations at the lower temperature usually employed for the oxidation reaction, for example silica more highly doped with fluorine than hitherto achieved.

This application is a continuation of application Ser. No. 757,963,filed July 23, 1985 now abandoned.

This invention relates to optical fibre manufacture and in particular,but not exclusively, to vapour phase deposition processes for use in theproduction of doped silica glasses, such as for optical fibre claddinglayers.

The dopant may be fluorine which significantly reduces the refractiveindex of silica.

According to one aspect of the present invention there is provided avapour phase deposition process involving a glass forming reaction forthe production of a doped glass from a reactant material, including thestep of preventing the occurrence of the glass forming reaction untilthe reactant material has been heated to a temperature higher than thatrequired for the glass forming reaction whereby to achieve theproduction of a doped glass of a composition not allowed fromthermodynamic considerations at lower temperatures.

According to another aspect of the present invention there is provided avapour phase deposition process for the production of a doped silicaglass from an oxidation reaction of a reactant material comprisingsilicon tetrachloride and a reactant including the dopant, wherein thereactant material is heated to a temperature at which exchange reactionoccurs between at least some of the silicon tetrachloride and thereactant including the dopant, which temperature is higher than thatrequired for the oxidation reaction, and wherein the oxidation reactionis prevented from occurring until after the reactant material has beenso heated whereby to achieve the production of a doped silica glass of acomposition not allowed from thermodynamic considerations at lowertemperatures.

According to a further aspect of the present invention there is providedan apparatus for use in inside tube vapour phase deposition of a dopedglass by means of an oxidation reaction for the production of a preformfor optical fibre manufacture, comprising means for rotatably supportinga substrate tube in use of the apparatus; means for heating a zone ofthe substrate tube oxidation reaction; means for traversing the heatingmeans along the length of the substrate tube; vapour train means forgenerating reactant vapours; means for supplying the reactant vapours tothe interior of the substrate tube, and means for supplying oxygen tothe interior of the substrate tube, the reactant vapours supply meansand the oxygen supply means being such that in use of the apparatus theoxidation reaction is prevented until after the reactant vapours havebeen heated to said temperature whereby to achieve the production of adoped glass of a composition not allowed from thermodynamicconsiderations at a lower temperature.

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 illustrates a graph of Δn (refractive index depression) versusflow rate of CF₂ Cl₂ as predicted by a computer model together withresults achieved experimentally;

FIG. 2 illustrates the equilibrium composition of various components ofthe reactant vapours in terms of variations in mole fraction withtemperature;

FIG. 3 illustrates the variation in the relative concentration of thefluorinating species [(SiCl₃ F )/(SiCl₃ F+SiCl₄)] with temperature, and

FIG. 4 illustrates schematically an inside tube vapour phase depositionprocess according to the present invention.

In the vapour deposition method for producing low loss optical fibres,layers of high purity glass are deposited on the inner wall of a silicasubstrate tube as the product of thermally initiated or plasma activatedvapour phase reactions. The present invention is concerned withthermally initiated reactions. The silica substrate tube is extensivelyprecleaned prior to mounting in a form of horizontal lathe. The interiorof the tube is connected to the output of a vapour train which generatesthe required quantities of source reagent vapours. High purity halidesource materials are vaporised and carried into the substrate tube withoxygen gas. There is no reaction under normal ambient conditions but atelevated temperatures, as produced by the flame of an oxy-hydrogen torchtraversing along the length of the tube and directed thereat to producea traversing hot zone, a chemical vapour phase reaction occurs to form amixture of oxides which are simultaneously deposited and fused as aglassy layer to the inner wall of the substrate tube. By traversing thehot zone uniformally along the tube, a uniform deposit is built up.

Deposition reactions are of the form

    SiCl.sub.4 (g)+O.sub.2 (g)→SiO.sub.2 (s)+2Cl.sub.2 (g) (1)

The deposition temperature required is a function of the fusiontemperature of the deposited glass. Pure silica requires a temperatureof 1900-2100K to be simultaneously deposited and fused, although theSiCl₄ +O₂ reaction can go to completion at 1200° C. (≃1500K). Thedeposition (with fusion) temperature reduces as the quantity of dopantincreases. For doped silica a mixed oxide glass may be produced byintroducing various dopants with the SiCl₄, for example GeCl₄ and POCl₃,in order to achieve a mixed oxide glass of SiO₂, GeO₂ and P₂ O₅ forfibre core material, for example, or BBr₃ to achieve a SiO.sub. 2/B₂ O₃mixed oxide for a cladding layer on such a mixed oxide core. Forfluorine doping of a cladding layer, therefore, the dopant sourcecompounds BBr₃ in the above example, is replaced by a suitable F sourcematerial. After deposition of the initial optical cladding material e.g.SiO₂ /F, the optical core is deposited, which may be solely SiO₂.

The coated tube is then collapsed into a rod preform by increasing thetemperature of the hot zone to approximately 2100° C. (2400K) whensurface tension forces cause the softening silica wall of the tube tocollapse upon itself and eventually seal the interior hole. Traverse ofthe hot zone then continues the collapse along the whole tube to producea rod preform which can then be pulled into a fibre.

Whereas the SiCl₄ +O₂ reaction is the most difficult of the SiCl₄,GeCl₄, POCl₃ and BBr₃ reactions, since the activation energy for thereaction is high and temperatures above 1400K are required to obtainappreciable reaction rates, the thermal decomposition of silicontetrafluoride is difficult. The reaction

    SiF.sub.4 +O.sub.2 →SiO.sub.2 +2F.sub.2

is thermodynamically very unfavourable, ΔG°₂₁₀₀, the standard freeenergy of reaction at 2100K (a typical reaction and depositiontemperature) being +768KJ/mole, as opposed to -158KJ/mole for SiCl₄ +O₂,and Kp the equilibrium constant being vanishingly small andapproximately zero, as opposed to 8512 for SiCl₄ +O₂. (ΔG°₂₁₀₀^(=-RTInKp)). Some deposition occurs when silicon tetrachloride is mixedwith the fluoride but oxidation to silica is considerably slower thanfor the chloride alone. (see for example J. Irven et al Optical Fibresby Plasma Augmented Vapour Deposition - Physics and Chemistry of GlassVol 21 No. Feb. 1 1980 p 48). It has been postulated that the reactionproceeds via the formation of an intermediate chloro-fluoro species e.g.

    3 SiCl.sub. 4+SiF.sub.4 ⃡4 SiCl.sub.3 F

for which ΔG°₂₁₀₀ is +58 kJ/mole and Kp=0.036, that is with a small butsignificant reaction in the forward direction. Then

    SiCl.sub.3 F+.sup.3 /.sub.2 O.sub.2 (g)→SiO.sub.1.5 F +.sup.3 /.sub.2 Cl.sub.2 (g)                                      (2)

where SiO₁.5 F represents a tetrahedrally bonded silicon with onefluorine and three bridging oxygen bonds. However little fluoride isincorporated even with large excesses of SiF₄.

A computer model based on thermodynamic predictions has shown goodagreement with experimental data for the fluorination of silica in theinside tube vapour deposited silica process. The computer model wasbased on the idea that the principle mechanism of fluorine incorporationinto silica based optical fibres is via reaction (2). The SiCl₃ F isgenerated via exchange reactions between SiCl₄ and fluorine or variousfluorine containing reagents (e.g. SiF₄, CF₂ Cl₂, CF₄, SF₆, BF₃) formingvarious proportions of all of the components of the SiCl_(4-n) F_(n)(n=0,1,2,3 or 4) series. The model uses the fact that reaction (1) goesto completion at 1200° C. (≃1500K) and the assumption that reaction (2)follows similar kinetics. FIG. 1 illustrates a graph of Δn versus flowrate of CF₂ Cl₂ as predicted by the computer model, experimental resultsbeing marked thereon showing good agreement therewith and thus tendingto confirm that SiCl₃ F is the prime precursor to fluorinated glass. (Δnis the refractive index depression relative to pure silica).

In considering a process where SiCl₄ and a fluorinating agent areprogressively heated from ambient temperatures in the presence of oxygenand if reaction (2) follows similar kinetics to reaction (1) as above,therefore, no fluorine exchange would be possible above 1200° C.(≃1500K) because no suitable silicon chloride species would be availablefor reaction. This implies that the ratio of SiCl₃ F:SiCl₄ in the gasphase, at the temperature at which the oxidation reaction occurs,determines the ratio of the SiO₁.5 F:SiO₂ in the deposited glass. FIG. 2shows the actual proportions of the chlorofluorosilanes (SiCl_(4-n)F_(n)) at different temperatures temperature from 300 to 2500K. It canbe seen that the ratio of SiCl₃ F:SiCl₄ increases with increasingtemperature. This is shown more clearly in FIG. 3 which shows theproportion of SiO₁.5 F which would be incorporated into the glass ateach temperature. It is assumed that only SiCl₃ F and SiCl₄ react withoxygen to form the glass and that the ratio of these gives the ratio ofSiO₁.5 F and SiO₂ in the deposited glass, the amount of fluorine in theglass being directly proportional to the refractive index depression Δn.As an illustration of FIG. 3, if one considers the oxidation reaction tobe essentially complete at 1500K, from the graph the fraction of SiO₁.5F in the glass would be approximately 0.07, whereas at 1800K thefraction would be approximately 0.15, that is a 300K temperature risewould double the fluorine incorporation. Hence if, for example, theoxidation reaction is delayed until the reactants reach a highertemperature than that at which oxidation occurs, exchange reactionsbetween the SiCl₄ and the fluorinating agents may take place first andthe proportion of fluorine in the deposit will be increased whenoxidation is eventually permitted.

The oxidation reaction may be delayed in the internal tube process byarranging a feed tube inside the substrate tube and progressively movingthe feed tube with the torch heating the substrate tube. Thisarrangement is shown schematically in FIG. 4. The substrate tube 1 isheated by a flame (indicated by arrow 2) from a torch (not shown)whereby to heat the substrate tube 1 on both sides axially of the end ofa feed tube 3. Oxygen is passed down the feed tube 3 and the otherreactants gases (SiCl₄ plus fluorinating reagent), with or without aninert carrier gas, are passed between the feed tube 3 and the substratetube 1. The feed tube 3 is positioned so that the oxygen only mixes withthe other reactant gases in the heated region, typically at temperaturesof 1500°-1600° C. (≃1800-1900K) and reacts in reaction zone 4. The gasflows may be exchanged i.e. the oxygen may flow between the substratetube and the feed tube with the reactant gases fed down the feed tube.

The method of increasing fluorine incorporation is not restricted to theinternal tube process and is applicable to other optical fibre processeswhereby the oxidation reaction is prevented until a higher temperaturethan would normally be the case in order to allow exchange reactions tooccur first. Similarly the technique is not restricted to fluorinedoping of silica glasses since other dopants may be similarlyincorporated by a process involving an oxidation reaction of silicontetrachloride, which oxidation reaction is prevented from proceedinguntil a temperature higher than would normally be the case in order toallow exchange reactions with other reactants to occur, for the purposeof producing doped silica glasses of compositions not allowed bythermodynamic considerations at the lower normal temperature. Whereasthe manufacture of optical fibres is described, the glass need not bein, or intended after further processing, to be in, optical fibre form,the low refractive index glass provided by the incorporation of fluorinein silica also being of use in other optical components.

Whereas the method of fluorine incorporation in silica described abovegenerates larger than usual quantities of SiCl₃ F in situ andimmediately prior to the oxidation reaction, alternatively the silicontetrachloride and a fluorinating agent may be preheated, to atemperature higher than required for the oxidation reactions in an inertgas, prior to direction into the substrate tube, the feed tube not beingnecessary in this case, in order to promote chemical exchange andproduce SiCl₃ F for direction to the reaction zone where it reacts withoxygen also directed into the substrate tube, rather than productionjust before it as described with respect to FIG. 4, the oxidationreaction thus being prevented from occurring until larger quantitiesthan usual of SiCl₃ F are present than those which normally occurwhereby to increase the fluorine dopant level in the deposited silica.

I claim:
 1. A thermally initiated vapor phase deposition process for theproduction of fluorine doped silica for optical fibre, comprising thesteps of heating silicon tetrachloride together with a fluorinatingagent to a first temperature whereby to produce an exchange reactiontherebetween, the reactant vapors resulting from the exchange reactionincluding silicon tetrachloride and silicon trichlorofluoride (SiCl₃ F),the ratio of silicon trichlorofluoride to silicon trichlorofluoride andsilicon tetrachloride in said reactant vapors increasing with saidincreasing values of said first temperature, and subsequently to saidexchange reaction combining oxygen with said reactant vapors to cause anoxidation reaction to occur to produce fluorine doped silica comprisinga mixed oxide of silica and SiO₁.5 F, which oxidation reaction can go tocompletion at a second temperature that is lower than the firsttemperature, the use of the first temperature for the exchange reactionwhich is higher than the second temperature needed for the oxidationreaction to go to completion causing the production of higher quantitiesof silicon trichlorofluoride than if the exchange reaction is carriedout at the second temperature and causing increased fluorine dopantlevels in the produced fluorine doped silica.
 2. A process as claimed inclaim 1, wherein the fluorinating agent is selected from the groupconsisting of SiF₄, CF₂ Cl₂, CF₄, SF₆ and BF₃.
 3. A process as claimedin claim 1, wherein the silicon tetrachloride and the fluorinating agentare heated to said first temperature together with an inert gas in orderto promote the exchange reaction.